Question

I am trying to dynamically set (not create, it already has to exist) a global ruby variable in a method. The variable name is determined from the passed symbol. What I am currently doing is the following:

def baz(symbol)
  eval("$#{symbol}_bar = 42")
end

$foo_bar = 0
baz(:foo)
puts $foo_bar # => 42  

But to me, this kind of feels very wrong. Is this the way to do this? Or can it be done differently? Also, I don’t know how evals perform in ruby. Does it run much slower than

$foo_bar = 42

Answer 1

If you can use an instance variable instead, there is Object#instance_variable_set.

  def baz(symbol)
    instance_variable_set("@#{symbol}_bar", 42)
  end

Note that it only accepts variable names that can be accepted as an instance variable (starting with @). If you put anything else in the first argument, it will return an error. For the global variable counterpart to it, there is a discussion here: Forum: Ruby

Either way, you also have the problem of accessing the variable. How are you going to do that?